os/user: `bad pointer in frame os/user.lookupUnixGid`

[bcmills]

#!watchflakes
post <- `bad pointer in frame os/user.lookupUnixGid`

(extracted from #55167 (comment))

I believe this is related to the os/user allocation changes in CL 449335 and/or CL 449316. (CC @ianlancetaylor @rsc)

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- `bad pointer in frame os/user.lookupUnixGid`

2022-12-08 03:49 linux-s390x-ibm go@b9747e0e os/user.TestLookupGroup (log)

runtime: bad pointer in frame os/user.lookupUnixGid at 0xc000104e00: 0x78
fatal error: invalid pointer found on stack

runtime stack:
runtime.throw({0x1767a6, 0x1e})
	/data/golang/workdir/go/src/runtime/panic.go:1047 +0x58 fp=0x3ffcf4fedf0 sp=0x3ffcf4fedc8 pc=0x50538
runtime.adjustpointers(0xc000104de8, 0x3ffcf4feee0, 0x3ffcf4ff2c0, {0x235b28, 0x2476e0})
	/data/golang/workdir/go/src/runtime/stack.go:632 +0x2b0 fp=0x3ffcf4fee50 sp=0x3ffcf4fedf0 pc=0x6cf00
runtime.adjustframe(0x3ffcf4ff1e8, 0x3ffcf4ff2c0)
	/data/golang/workdir/go/src/runtime/stack.go:674 +0x10e fp=0x3ffcf4fef00 sp=0x3ffcf4fee50 pc=0x6d01e
...
	/data/golang/workdir/go/src/os/user/cgo_lookup_unix.go:184 +0x66 fp=0xc000104da0 sp=0xc000104d40 pc=0x130866
os/user.lookupUnixGid(0x0)
	/data/golang/workdir/go/src/os/user/cgo_lookup_unix.go:120 +0x8c fp=0xc000104e58 sp=0xc000104da0 pc=0x13042c
os/user.lookupGroupId({0x177dee, 0x1})
	/data/golang/workdir/go/src/os/user/cgo_lookup_unix.go:113 +0x78 fp=0xc000104e88 sp=0xc000104e58 pc=0x130368
os/user.LookupGroupId(...)
	/data/golang/workdir/go/src/os/user/lookup.go:64
os/user.TestLookupGroup(0xc000083520)
	/data/golang/workdir/go/src/os/user/user_test.go:110 +0x120 fp=0xc000104f68 sp=0xc000104e88 pc=0x131e00
testing.tRunner(0xc000083520, 0x17be40)

— watchflakes

[gopherbot]

Change https://go.dev/cl/456121 mentions this issue: os/user: zero-initialize C structs returned to Go

[bcmills]

Marking as release-blocker for Go 1.20 because it is a regression in Go 1.20 (CL 449335) and can cause crashes (or GC-related memory errors) in any cgo-enabled program that uses the os/user package.